Electrophysiological and digital video microscopic techniques are used primarily to elucidate the development, differentiation and cellular distribution of physiological properties expressed by embryonic and postnatal mammalian CNS neurons. Electrical studies involve direct, continuous high-fidelity amplification of ion fluxes generated either in single cells or patches or in pairs of cells in culture. Optical recordings include indirect measurements of membrane potential or cytoplasmic Ca2+ (Ca/c2+) in small populations (50-100) of cultured cells. Principal findings include: 1) regenerative Na/o+-dependent action potentials and underlying voltage-dependent currents are expressed early in embryonic telencephalic and spinal neurons, when most of the cells are still actively proliferating; 2) electrical and dye coupling among cortical cells recorded in situ disappears during embryogenesis; 3) undifferentiated, large-conductance K+ channels appear in the proliferating period with heterogeneous properties including unstable openings, variable voltage-dependence, Ca2+ sensitivity and kinetics; 4) K+ channel properties differentiate during the rest of the embryonic period; 5) micromolar GABA activates Cl- channels with heterogenous properties in spinal cord cells beginning during the proliferative period; 6) activation of Cl- channels depolarizes virtually all embryonic neurons acutely recovered from spinal and supraspinal regions; 7) routine culture methods show that the depolarizing effects of GABA disappear progressively over a period of several weeks; 8) this transformation in polarity of GABA's Cl--dependent voltage signal can be hastened by co-culturing cells on astrocytes or by using routine methods combined with medium conditioned by astrocytes; 9) initially GABA is released from embryonic neurons in a continuous, tonic manner capable of polarizing cells near E/Cl before it is discharged in transient pulses that generate synaptic-like events; 10) cultured embryonic hippocampal neurons secrete and receive GABA in a new form of fast communication ("cismission," same-sided signalling); 11) "cismitting" GABAergic neurons discharge GABA in transient pulses at negative potentials and in a continuous mode at depolarized potentials; 12) exogenous GABA sticks to the surface of embryonic neurons long after the application is over, as if the exposed surface promotes an unstirred equilibrium of GABA that randomly activates a perseverating Cl- signal; 13) kinetic components of open Cl- channels activated by GABA shorten during development in cells dissociated from spinal and supraspinal regions, as GABA becomes inhibitory in function.